We use porous materials to strengthen the evaporation process by increasing the evaporation area.

2.2.1 Evaporation behaviors with and without porous materials

As we can see from the Fig.5, it is clearly that the technique combined with porous materials can strengthen the evaporation process greatly.

During the experiments, one side of porous materials is dipped into the seawater, and then seawater rises along the materials based on the capillary phenomena. We choose two kind of materials, gauze and non-woven fabric band, which have same area (95×70=6650mm2). And we also learn that non-woven fabric band is better than gauze for evaporation process. Compared with the process with and without porous materials, the efficiency aggrandizes remarkably. So, it is feasible that solar chimney dryer combined with porous materials. And different material has different effect to evaporation process because of different characters.

2.2.2 Evaporation behaviors at different wind velocities with porous materials

As we can see from the Fig6 and Fig7, it is clearly that the evaporation process is strengthened when the wind velocity increases, both for gauze and non-woven fabric band.

The wind velocity is an important factor to the evaporation process because it can remove the air above the surface of seawater quickly, which can enhance the humidity gradients. Compare with the evaporation process, the efficiency for evaporation increases when porous materials are used. And it is obviously effectual when wind velocity changes from 0.55m/s to 1.68m/s (45 C). The encumbrance effect of seawater mass transfer from porous material inside to surface is not very strong because the thickness of porous materials is very thin.

2.2.3 Evaporation behaviors with different pieces of porous material

We investigated the evaporation process with different pieces of porous materials at 0.55m/s to 1.68m/s (45 C). Fig.8-11 shows evaporation behaviors with different pieces of gauze and non-woven fabric band.

The evaporation area increases firstly with the number pieces of porous material increases but still gets a decrease with the further more pieces, except for the tendency in the Figure 11, which revealed that the more pieces of porous materials does not mean the quicker evaporation velocity. Although the increasing of evaporation area makes more water evaporated per minute, but the larger evaporation area will weaken the tendency and finally results in a smaller evaporation velocity. So it is another important factor to use porous materials in solar chimney dryer.

2.2.4 Effect of different location of porous material

We also investigate the effect of the mounted angles between the porous material plane and the wind, which were chosen as 0°,45° and 900. From Figure 12, it is obviously seen
that the effect to evaporation declines when angle increases, because it will generate more resistance to the wind. It is reasonable according to the hydrodynamics.

3. Conclusion:

1. The temperature is one of important influence factor to evaporation. The evaporation process can be strengthened greatly when the temperature increases among the experimental conditions.

2. The airflow is another important factor to the evaporation. The evaporation process can be also strengthened greatly when the airflow velocity increases. But it will be an encumbrance to the evaporation when the airflow is too high. The reason is that the high airflow velocity is a strong force to evaporation of seawater, and then a layer of salt appears to baffle the transfer of water from the main body of seawater to the air. So the airflow velocity should be controlled below 1.68 m/s, in case of affecting the evaporation process.

3. The combination of porous materials makes for the evaporation process greatly. As mentioned above, the gauze is dipped into the seawater and the seawater rises along the materials based on the capillary phenomena, which increases the evaporation surface. The additional function varies with the property of the porous materials. We can see clearly from the Fig.5, the non-woven fabric band is more effective than gauze because of the adsorption ability. The study of porous materials related to this technique will be investigated in the following days.

In conclusion, the solar chimney dryer combined with porous materials was a perfect technique to drying process. In this technique, temperature, airflow velocity, porous materials were important factors to the evaporation process. In this paper, we used wind tunnel investigating the drying process to simulate the situation in the solar chimney dryer and to study the influencing factors. In the following days, we would perform the study of the relationship between the wind tunnel and solar chimney.

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